AGU journal highlights -- May 29, 2009

The following highlights summarize research papers that have been published in Geophysical Research Letters (GRL) or the Journal of Geophysical Research - Atmospheres (JGR-D).

In this release:

Avalanches of electrons may give thundercloud insights

Positive feedback hint between tropical cyclones and global warming

Moon dust stickiness depends on Sun angle

Probing clouds' roles in global electric circuit

Satellites observe Amazon basin water storage and runoff

Waves in Earth's radiation belt get mapped

Anyone may read the scientific abstract for any of these papers by clicking on the link provided at the end of each Highlight. You can also read the abstract by going to http://www.agu.org/pubs/search_options.shtml and inserting into the search engine the full doi (digital object identifier), e.g. 10.1029/2008JD011386. The doi is found at the end of each Highlight below.

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1. Avalanches of electrons may give thundercloud insights

Understanding thundercloud electrification and lightning initiation is challenging because the ranges of electric potential and spatial extents of electric fields inside thunderclouds are not known. Directly measuring electric fields in thunderclouds is challenging--active regions of storms can cover many cubic kilometers with violent weather conditions, making it difficult to operate balloons and aircraft. Dwyer et al. hypothesize that remote sensing of thundercloud electrostatic fields can be obtained through monitoring runaway electrons, which are produced when the rate of gain of energy by electrons moving through an electric field exceeds the rate of loss of energy from ionizing the air. Such runaway electrons trigger other runaway electrons, resulting in an exponentially growing avalanche of runaway electrons moving through the storm system. The authors propose that radio frequency emissions produced by these avalanches can be monitored to map the magnitudes and directions of the electrostatic field within specific sections of a thundercloud. These radio frequencies are present only when the storm is bombarded by cosmic ray extensive air showers, allowing scientists quickly to identify particular storms that are favorable to the proposed remote sensing techniques.

Tropical cyclones could be a significant source of the deep convection that carries moist air upward to the stratosphere, where it can influence climate, according to Romps and Kuang. Using 23 years of infrared satellite imagery, global tropical cyclone best-track data, and reanalysis of tropopause temperature, the authors find that tropical cyclones contribute a disproportionate amount of the tropical deep convection that overshoots the troposphere and reaches the stratosphere. Tropical cyclones account for only 7 percent of the deep convection in the tropics, but 15 percent of the convection that reaches the stratosphere, they find. The authors conclude that tropical cyclones could play a key role in adding water vapor to the stratosphere, which has been shown to increase surface temperatures. Because global warming is expected to lead to changes in the frequency and intensity of tropical cyclones, the authors believe their results suggest the possibility of a feedback mechanism between tropical cyclones and global climate.

Lunar dust, which is thought to be the prime environmental hazard on the Moon, can damage scientific instruments and harm the health of astronauts who inhale it, but its adhesive properties have not been well understood. Analyzing data from the matchbox-sized dust detector experiments deployed by astronauts on the Apollo 11 and 12 missions in 1969, O'Brien finds that the electrostatic forces that make lunar dust stick to surfaces vary with solar elevation angle. This is because dust particles become positively charged through photoelectric effects excited by solar ultraviolet radiation and X rays, so more intense direct sunlight increases the electrostatic forces that make the dust adhesive. O'Brien's analysis shows that after dust had collected on the detector and as the solar incidence angle decreased over the course of the lunar day, the electrostatic forces holding the dust to the detector's vertical silicon surface weakened, and the dust began to fall off due to gravity. The author suggests that on future Moon missions, a sunproof shed could provide a dust-free working environment for astronauts.

A semicontinuous current flow has been measured above electrified clouds. Called the Wilson current, this phenomenon has long been considered a critical component of the global electric circuit; however, only a few studies have directly investigated this current, yielding only a few dozen measurements. Mach et al. greatly expand on this sparse collection of measurements. Using conductivity and electric field sensors mounted on high-altitude aircraft, they determined current densities along 850 overflights of electrified clouds. Using these data, which represent more than a decade of observations of land-based and oceanic storms, the authors estimate the Wilson current for each of the clouds overflown. They find that one third of the clouds studied did not have lightning yet still generated significant Wilson currents, confirming that such "shower" clouds do play a critical role in the global electric circuit. They also find that 7 percent of storms produced current flows that were opposite in polarity from the typical thunderstorms, diminishing the contribution storms make to the global electric circuit. The results of this paper will contribute to a better understanding of the global electric circuit.

Scientists would like to better understand the physical processes in Amazon hydrological systems. To explore the water storage and dynamics in the Amazon basin, Han et al. use 4 years of data from the two Gravity Recovery and Climate Experiment (GRACE) satellites, which measure mass distribution on Earth's surface through instantaneous measurements of the changes in the distance between the satellites. Water stored in the Amazon basin affects mass distribution and thus can be monitored by the GRACE satellites. The authors find that soil water explains about half of the observed changes in intersatellite distance; surface and subsurface runoff explains the rest. By comparing river runoff routing simulations with GRACE data for the Amazon region, the authors find that the overall effective runoff velocity for the entire Amazon basin was about 30 centimeters per second (about one foot per second), with significant seasonal variation. They conclude that incorporating GRACE data can help improve routing schemes in large-scale land surface models.

Title: Dynamics of surface water storage in the Amazon inferred from measurements of inter-satellite distance change

Chorus waves, a type of electromagnetic emission generated by electrons in Earth's radiation belt, play an important role in both accelerating and removing the energetic radiation belt electrons that can disrupt satellite electronics and disturb communications with ground-based operators. To improve understanding of the origin and location of chorus waves, Li et al. use data from NASA's five Time History of Events and Macroscale Interactions during Substorms (THEMIS) satellites to create a new map of their distribution. The authors find, in agreement with previous studies, that nightside chorus occurs only near the equator, whereas dayside chorus extends to higher latitudes. In addition, they observe that amplitudes of chorus waves depend strongly on geomagnetic activity. The most important new finding, the authors note, is that at a distance of more than 7 Earth radii (about 45,000 kilometers) on Earth's dayside, moderate chorus is present more than 10 percent of the time (a much higher occurrence rate than on the nightside) and persists even during periods of low geomagnetic activity. The authors believe that the new information could provide additional clues about the origin of dayside and nightside chorus waves.

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